Thermal processing is required in the fabrication of silicon and other semiconductor integrated circuits formed in silicon wafers or other substrates such as glass panels for displays. The required temperatures may range from relatively low temperatures of less than 250° C. to greater than 1000° C. or 1300° C., for example, and may be used for a variety of processes such as dopant implant annealing, crystallization, oxidation, nitridation, silicidation, and chemical vapor deposition as well as others.
For the very shallow circuit features required for advanced integrated circuits, it is greatly desired to reduce the total thermal budget in achieving the required thermal processing. The thermal budget may be considered as the total time at high temperatures necessary to achieve the desired processing temperature. The time that the wafer needs to stay at the highest temperature can be very short.
Rapid thermal processing (RTP) uses radiant lamps. Such lamps have an on/off switching time of at least a half second and therefore cannot avoid heating the entire bulk of the wafer. As a result, the anneal time must be limited to avoid exceeding the thermal budget. Pulsed laser annealing using very short (about 20 ns) laser pulses is effective at heating only the surface layer and not the underlying wafer, thus allowing very short ramp up and ramp down rates.
A more recently developed approach in various forms, sometimes called thermal flux laser annealing or dynamic surface annealing (DSA), is described in U.S. Pat. No. 6,987,240, issued Jan. 17, 2006 by Jennings et al. and incorporated herein by reference in its entirety. This DSA apparatus uses CW diode lasers to produce very intense beams of light that strike the wafer as a thin long line of radiation. The line is then scanned over the surface of the wafer in a direction perpendicular to the long dimension of the line beam.
In DSA processing, temperature measurement is desirable because the amount of light coupled into the wafer strongly depends upon the surface structure already formed in the wafer. Furthermore, light source conditions may vary somewhat. Wide-angle pyrometers are generally used in RTP apparatus to monitor large portions of the wafer. Such pyrometers are generally inappropriate for the focused laser beams of DSA apparatus the irradiate only a small area of the wafer at any time, leaving the bulk of the wafer near ambient temperature.
In DSA processing, there is a need to compensate for absorption and reflectivity variations across the semiconductor substrate surface and variation in output of the laser array across the thin line of radiation. For example, variation in reflectivity and absorption of thermal energy at the semiconductor device level for devices which are comprised of features that have varying absorption and reflectivity of said thermal radiation. Such features are many micrometers to submicrometers in dimension.